The present invention relates to an apparatus for detecting the occurrence of misfire in an internal combustion engine.
As a means for detecting the occurrence of misfire in an internal combustion engine there is known as disclosed in the Japanese Patent Provisional Publication No. 61-258955 where, under the fact that the occurrence of misfire can disturb the complete combustion within combustion chamber of the engine to lower the engine speed, the engine speed is detected at at least two points in the one-ignition cycle to obtain the variation of the engine speed which is in turn compared with a misfire detection value set on the basis of the state of the engine so that the occurrence of misfire is decided when the engine speed variation exceeds the misfire detection value. In this misfire detection apparatus the misfire decision is effected on the assumption that, when the misfire does not occur, the average speed .omega..sub.n at every ignition period (combustion period) is always constant as illustrated in FIG. 8. In FIG. 8, the dotted line shows the instantaneous speed characteristic of the engine and the solid line shows the average speed .omega..sub.n characteristic within each ignition period. However, in the case of a V-type internal combustion engine in which the cylinders are disposed about the crank shaft so as to have a V-configuration, since the connecting rod for connecting the piston to the crank shaft is disposed to have a predetermined angle with respect to the crank shaft, even if the combustion normally occurs, the average speed .omega..sub.n does not become constant due to the inertial force of each cylinder and the like but the average speed .omega..sub.n varies at every 360.degree. CA period as illustrated in FIG. 4. FIG. 4 shows the average speed .omega..sub.n characteristic of each of the cylinders of a V-type 6-cylinder internal combustion engine in which 6 cylinders are disposed to form a V-configuration by sets of 3 cylinders. For example, the average speed .omega..sub.n obtained when the third cylinder takes the explosion stroke is plotted at the top dead center position (#3TDC position) of the third cylinder on the horizontal axis. Similarly, in the case of an in-line type internal combustion engine, there is possibility that the aforementioned 360.degree. CA period variation occurs due to the difference between the crank angle sensors in the manufacturing process, the mechanical looseness of the crank shaft and others. Accordingly, when the average speed .omega..sub. n variation as illustrated in FIG. 4 occurs, there is the possibility that a decision error appears regardless of no occurrence of misfire.